1. Field pf the Invention
This invention generally relates to color purity correction in color television receivers, and more particularly is directed to an improvement in system for correcting beam landing in color cathode ray tubes which tends to be harmfully affected by the earth's magnetic field.
2. Description of the Prior Art
Generally, in the color cathode ray tube of a color television receiver, the current values of independent three electron beams, which may impinge upon three phospors for emitting three primary colors or red, green and blue colors (therefore, three electron beams will be hereinafter referred to as R-beam, G-beam and B-beam, respectively), are controlled to adjust the emitting intensities of three colors to thereby reproduce an image of arbitrary color. In this case, if the R-beam, G-beam and B-beam are not landed on the corresponding red, green and blue phosphors correctly, the color of the reproduced picture is deteriorated. For example, if the R-beam impinges upon the green phosphor, the fidelity of the color television receiver for reproducing color is spoiled. In general, the electron beams in a color cathode ray tube pass through an area in which horizontal and vertical deflection magnetic fields present, and thereafter advance substantially straight for the R, G and B-beams to impinge upon the predetermined red, green and blue phosphors on the inner surface of the face panel of the color cathode ray tube respectively. In fact, however, as well known, the electron beams are affected bu the earth's magnetic field, then deflected reluctantly and reached to positions displaced from the proper positions on the inner surface of the face panel in the cathode ray tube to become such a state that the respective electron beams do not impinge upon the correct phosphors or a so-called "mislanding", which deteriorates the color of a reproduced picture as described previously is caused. Since the influence of the earth's magnetic field becomes different in accordance with the position where the color television receiver is situated and the direction of the face panel of the cathode ray tube, the mislanding of electron beams also become different in accordance with the located position of the television receiver and the direction of the face panel.
In the art, there is proposed the system in which a coil is located around the funnel portion or peripheral portion of a cathode ray tube and a predetermined value of DC current is supplied to the coil to establish DC magnetic field so as to cancel the earth's magnetic field affecting the electron beams badly. However, with such a prior art system which produces the DC magnetic field as the correction magnetic field, it is difficult to produce correction magnetic field which may suitably follow up the influence of the earth's magnetic field which may suitably follow up the influence of the earth's magnetic field which influence changes in accordance with the direction of a cathode ray tube, and accordingly the mislanding can not be compensated for sufficiently.